i have a following situation.
Client is migrating from some 3rd party mail to Office 365. Currently using POP3. Autodiscover is pointing to Office 365.
Server 1: 3rd party, registered mydomain and mailbox with email@example.com. MX pointing to it.
Office 365 Exchange, registered mydomain.com and mailbox for firstname.lastname@example.org
What i usually do, for client with few users, is to configure Outlook to have both mailboxes (accounts), one for POP3 on 3rd party mail and second via autodiscover on Office 365, for (email@example.com). This works if you have Outlook 2010 or 2013, and i am sure it worked for Outlook 2016 some time ago.
You open Outlook and simple get mailbox for firstname.lastname@example.org (1) (POP3) and email@example.com (o365)
After that, i migrate MX, and inform users to start using new mailbox tomorrow, and then migrate old mailbox to new one.
Problem is now, when you install Outlook 365, when you try to add account for exchange mailbox on o365, Outlook is going to report that account already exists, and you cannot add it.
Is there some workaround on this? Why it does not work as good as it was before :)